AD9957 Analog Devices, AD9957 Datasheet - Page 14

• Manufacturer Part Number: AD9957
• Description: 1 GSPS Quadrature Digital Upconverter
• Manufacturer: Analog Devices

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DataSheet
AD9957
Knowledge of the frequency response of the half-band filters is
essential to understanding their impact on the spectral proper-
ties of the input signal. This is especially true when using the
quadrature modulator to upconvert a base band signal contain-
ing complex data symbols that have been pulse shaped.
To better understand this concept, consider that a complex
symbol is represented by a real (I) and imaginary (Q) compo-
nent. Thus, two digital words are required to represent a single
complex sample of the form: I+jQ. The sample rate associated
with a sequence of complex symbols will be referred to as f
BOL
rate must necessarily be increased by some integer factor, M (a
consequence of the pulse shaping process). This new sample
rate with be referred to as f
by:
Thus, f
to the input of the first half-band filter in both the "I" and "Q"
signal paths. NOTE: This rate is not to be confused with the rate
at which parallel data is supplied to the AD9957( f
equal to 2f
Typically, pulse shaping is applied to the base band symbols via
a filter having a raised cosine response. In such cases, an excess
band width factor (α) is used to modify the band width of the
data where 0 ≤ α ≤ 1. A value of 0 causes the data band width to
correspond to ½f
width to be extended to f
tionship between α, the band width of the raised cosine re-
sponse, and the response of the first half-band filter.
f
IQ
. If pulse shaping is applied to the symbols, then the sample
4
=
U
Mf
IQ
.com
is the rate at which complex samples must be supplied
SYMBOL
IQ
.
SYMBOL
, while a value of 1 causes the data band
SYMBOL
IQ
, and is related to the symbol rate
. Figure 10 illustrates the rela-
DATA
), which is
Rev. PrF | Page 14 of 38
SYM-
The responses in Figure 10 are shown for the specific case of
M=2 (the interpolation factor for the pulse shaping operation).
In cases for which M>2, the location of the f
band response portion of the diagram shifts to the right, as it
must remain aligned with the corresponding Mf
the frequency axis of the raised cosine spectral diagram. How-
ever, if f
proportionally.
The result is that the raised cosine spectral mask always lies
within the flat portion (DC to 0.4f
of the first half-band filter, regardless of the choice of α so long
as M>2. Therefore, for M>2, the first half-band filter has abso-
lutely no negative impact on the spectrum of the base band
signal when raised cosine pulse shaping is employed. However,
for the case of M=2, a problem can arise. This is highlighted by
the shaded area in the tail of the α=1 trace on the raised cosine
spectral mask diagram. Notice that this portion of the raised
cosine spectral mask extends beyond the flat portion of the
half-band response and will cause unwanted amplitude and
phase distortion as the signal passes through the first half-band
filter. To avoid this, simply ensure that α≤0.6 when M=2.
Programmable Interpolating Filter
The Programmable Interpolator is implemented as a CCI filter
with a low-pass frequency characteristic. It is programmable by
a 6-bit control word, giving a range of 2× to 63× interpolation.
Band Width
Nyquist
Half-band filter
= 1
= 0
Figure 10. Effect of the Excess Band width Factor (α)
response
½f
½f
IQ
SYMBOL
SYMBOL
shifts to the right, so does the half-band response,
Raised cosine
spectral mask
PRELIMINARY TECHNICAL DATA
0.4f
= 0.5
IQ
f
f
SYMBOL
SYMBOL
½ f
IQ
Typical spectrum of a random symbol sequence
Input sample rate of 1
half-band filter
IQ
) of the pass band response
oversampled pulse shaping
2f
2f
st
SYMBOL
SYMBOL
Sample rate for 2x
f
IQ
IQ
Output sample rate of 1
point on the half-
half-band filter
SYMBOL
point on
4f
3f
SYMBOL
SYMBOL
st
2f
IQ
f
f
f